1. Field of the Invention
This invention relates generally to an architecture for controlling the impedance and force provided by a series elastic actuator and, more particularly, to an architecture for controlling the impedance and force provided by a series electric actuator that includes a position sensor for determining the position of a motor shaft at one end of a spring, a position sensor for determining the position of a load at an opposite end of the spring, and an embedded high-speed processor that receives the measurement signals from the sensors and controls the orientation of the motor shaft to provide torque on the spring to control the orientation of the load, where the embedded processor receives torque reference commands from a remote controller.
2. Discussion of the Related Art
A series elastic actuator (SEA) employs a spring or other elastic element between a motor and the output of the actuator to transmit motion of the motor to motion of the actuator output. The deflection of the spring is typically used to measure the torque that is applied to the actuator output. SEAs are typically used in robots where the actuator is used to move the robot joints and links.
There are two main benefits to using SEAs. First, the relatively high compliance of the elastic element with respect to the motor transmission decouples the actuator output from the motor at high frequencies. This reduces the high-frequency passive inertia of the robot link, even when the motor and motor transmission have a large inertia. As a result, the lower high-frequency passive inertia makes SEA-driven robots safer around humans.
A second advantage of SEAs is an improved ability to control forces applied by the actuator and, therefore, to control actuator impedance. When the elastic element has a relatively high compliance with respect to the environment, then the sensitivity of the actuator force to small changes in motor position is reduced. As a result, it is easier to control applied actuator force using a position-controlled motor. Also, when the spring constant of the elastic element is known precisely, it is possible to measure actuator output forces by measuring spring deflection. This can eliminate the need for direct measurement of applied forces.
Although most mechanical realizations of SEAs are similar, there are several different approaches to SEA control. Most previous work on SEA control focuses on methods for controlling the SEA output force. One of the earliest SEA control strategies is essentially a PID (proportional-integral-derivative) controller on force error. Applied force is measured using a strain gauge mounted on the elastic element. This is compared to a force reference and the difference on this error.
Another approach to SEA control uses an internal motor position or velocity controller that is cascaded with the force controller. The force controller calculates a force error by differencing applied force and the force reference. A PD controller acts upon the force error and calculates a desired motor velocity. This velocity reference is input to the motor velocity controller. The motor velocity controller is implemented as a PID controller with a differentiator in its feedback path.